Section 4: Manual Setup Measurement Parameters
67
Window
Measured data is usually not directly used to generate a spectrum
but is first modified using a 'window' function. Windowing
compensates for known limitations of the mathematical process
(FFT) that transforms the data into a spectrum. Multiplying data by a
window function prevents spectral lines from smearing or leaking into
one another, thereby making the data block appear like a complete
wave. The Hanning window is normally used for this purpose. If the
Rectangular window is used the data will not be modified.
Setting the Number of Spectral Lines
• From the Set Parameters Menu press to toggle the
number of spectral lines.
The number of spectral lines ranges from 400 to 3200 (6400 for the
vb3000).
The resolution of the spectrum increases with the number of spectral
lines used i.e. the more spectral lines the more information the
spectrum contains. However, the more spectral lines used, the
longer the measurement takes and the more memory is used to
store the spectrum. Use 6400 lines only when required e.g. when
you need to distinguish between two closely-spaced vibration
frequencies or when the Fmax is very large. For coast-down or run-
up measurements a setting of 400 lines is recommended.
Note: When taking a dual channel recording the number of spectral
lines is reduced to 1600 (3200 for the vb3000).
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